1. Technical Field
The present invention relates in general to pulse width modulation circuits, and in particular to pulse width modulation circuits used in conjunction with circuits for controlling heavier and noise sensitive electrical loads, such as laser driver power circuits, on printed circuit boards and like substrates, in a highly accurate manner.
2. Discussion
Pulse width modulation (PWM) circuits have been in use for over forty years, and have grown in popularity and widespread use significantly in the last 30 years or so. In many applications, the accuracy of the timing of the individual pulse width modulation signals is not a particular concern, especially in some circuit applications like motor drive circuits, that will be discussed below. However, in a variety of precision circuit applications, where having a stable duty cycle of the individual pulses of a PWM pulse train is important, there have been continuing attempts to improve and refine duty cycle stability of such circuits and/or reduce their susceptibility to external or internal interferences. Nonetheless, there are at least noise or interference problems that appears to have been not solved by those PWM circuits that have been seen. Further this particular noise or interference problem is frequently present on PWM circuits implemented on printed circuit boards, particularly if there is ground-loop feedback among the control, comparator and/or power circuits. This particular problem will now be explained with reference to FIGS. 1 and 2.
FIG. 1 shows the conventional design of a pulse width modulation (PWM) circuit 20, which has a comparator 22 provided with two inputs, namely noninverting input 24 and inverting input 26, and an output 28. One of the inputs, such input 24, is connected to a triangle wave source Vtrwv(t), while another input 26 is connected to a control signal, Vcntr(t). As shown in FIG. 2A, the triangle waveform may take that the form of a conventional symmetrical rising and falling shape 34 with a generally uniform frequency, or it may take the form a conventional sawtooth waveform (not shown), also with a generally uniform frequency, depending on the design of the comparator circuit and/or the waveform generator voltage. The control signal, Vcntr(t), may vary with time, and in FIG. 2A, is represented by a straight line 36. A resulting PWM signal, Vout(t), is generated at output 28 of comparator 22. This PWM signal is represented by waveform 38 in FIG. 2B. The maximum peak-to-peak value of the triangle wave voltage on input 24 is within the input voltage range of comparator 22, and it is usually at 3 to 5 volts for a comparator powered by a 5 volt power supply.
As shown in FIG. 2A, when an interference voltage, Vint, is added to the control signal input, it will falsely trigger the comparator, and the trigger error in time caused by this interference is: Te=Vint/SR, where SR is the slew rate of the triangle wave voltage at another input of the comparator, its unit of measure being V/t, that is volts/time. FIG. 2 shows that when this interference signal Vint is added on top of the control input signal Vcntr(t), for example, it results in an earlier switching of comparator 22 from its high state to its low state, which means that this process of adding interference produces a trigger error Te, as shown in FIG. 2B. In other words, the output signal Vout(t) at output 28 is falsely triggered prematurely by the additive interference signal. Vint of a negative-going interference voltage impressed for any reason upon the control signal Vcntr would also result in false triggering in an opposite or time-delayed sense. In both cases, this error in time will be seen as a noise at the output voltage which will be proportional to the time error Te.
The interference signal or signals may arise from a variety of possible sources, and the severity of this noise may vary depending the specific circuits used, the noise source, the lay-out of circuit components, the routing of conductors and/or ground plane configurations, provisions that are made for shielding circuit components, leads and/or wires from electromagnetic induction, external noise, and the like. In a number of applications, for example, motor drive circuits, slight errors in the timing of individual pulses within a train of digital pulses likely will not have any significant effects. Moreover, in many automatic servo systems which use PWM drive signals, the closed loop nature of the control system automatically compensates for repeated or continuous noise that produces a steady state timing error in the length of individual square wave pulse in a PWM drive signal pulse train. However, in other systems, such as high-speed digital communications systems, including laser-based fiber optic systems which use electro-optical interfaces and PWM-based laser driver circuits, noise may lead to transposition errors in the state of the digital information and/or may result in the need to reduce the operational speed of the circuitry to take into account that interference signals will result in the time-shifting of individual pulses of the laser driver circuits in the manner described in connection with the waveforms shown in FIG. 2.
A number of earlier patents relating to PWM circuits have shown a variety of techniques to improve the accuracy of and/or the immunity of such circuits from noise or other forms of interference or variations in performance. Such patents include the following:
The foregoing patents do help show that state of the art with respect to the use and understanding of PWM circuits is well-developed, and that a number of steps have been taken over the years to combat noise and/or interference and/or transient condition problems encountered with PWM circuits used in a variety of applications. Accordingly, all of these patent references are hereby incorporated by reference as helping show what is now generally known in the field of electronic PWM circuits and taught in various U.S. patents. However, none of these patents appears to address how to solve or substantially reduce the interference or noise problem of the type described above with regard to FIGS. 1 and 2.
Accordingly, there is a continuing need to develop highly accurate and low noise pulse width modulation circuits that are less susceptible to altered timing of the individual pulses within a train of repetitive pulses of a PWM drive system. There is also a need to provide for PWM circuits which produce more accurate and stable timing of individual pulses even in the presence of interference signals impressed upon the input control signal, no matter what the source of the noise or interference.
Accordingly, it is a primary object of the present invention to provide an circuit apparatus and method which helps substantially reduce the adverse timing effects of interference signals imposed upon either the triangular waveform input signal or the input control signal supplied to a PWM controller. A related object of the present invention is to achieve this object without the use of exotic new control circuits, or the use of extra-high precision components.
Still another object is to develop a method and circuits which substantially solve or reduce the adverse noise/timing problem created by interference imposed upon the input leads to the comparator circuits of PWM controllers, particularly PWM controllers implemented with one or more printed circuit board designs involving power switching devices. A related object of the present invention is to advance the state of the art with respect to PWM methods and circuits which are particularly useful in high-speed data transmission applications, including but not limited to PWM-based drivers for diode lasers and other lasers used in optical telecommunication and instrumentation applications, including but not limited to fiber-optic communications systems and networks.
In light of the foregoing problems and in order to fulfill one or more of the foregoing objects, there is provided, in accordance with a first aspect of the present invention, a highly accurate, stable and low noise pulse width modulation circuit that features accurate timing in the face of an interference signal being imposed upon a control signal input thereto that is representative of the duty cycle of the desired PWM output. The PWM circuit is comprised of a first section which may be a digital or switching section, and second or analog section. The first section has a comparator provided with at least first and second inputs and an output, one of the inputs being a noninverting input and the other input being an inverting input. The output produces a PWM signal when proper signals are applied to the first and second inputs of the comparator. The first input signal for this section comes from the output of the analog section, and the second input signal may simply be a steady reference signal, such as a percentage of the power supply voltage.
The second or analog section of the PWM circuit includes an operational amplifier provided with first and second inputs and an output, one of the inputs being a noninverting input and the other input being an inverting input. The output of this op amp produces a substantially on-and-off signal, preferably in the form of a trapezoidal signal. The output of the operational amplifier of this analog section is connected to one of the inputs of the comparator of the switching section.
The second section preferably has signal adding means for combining first and second input signals, and providing the summed signal to at least one of the inputs of the operational amplifier of the analog section. The first input signal is connectable to a triangular wave source that preferably is highly stable and serves as the source of basic frequency of the PWM output signal. The second input signal is connectable to a control input signal. The analog voltage or value of this signal is generally proportional to the desired duty cycle of the PWM signal to be produced by the output of the comparator.
In operation, the control signal is summed and amplified in combination with the first input from the triangular wave source by the operational amplifier, whereby the output signal produced by the operational amplifier is a trapezoidal waveform. That output signal is delivered to and is in communication with at least one input of the comparator of the switching section.
The gain of the operational amplifier determines the rise time of the trapezoidal output signal. This gain should be set to at least five, and is preferably in the range of about ten to one hundred, although, depending on just how fast the sweep frequency of the PWM signal is, the gain may be set to a few hundred or even one thousand or more. The net result is that the fast rise time or slew rate of the op amp causes the timing of the PWM output signal to much more accurately follow the timing of the input control signal, as best illustrated and explained in connection with the circuit of the present shown in FIG. 3, and the waveforms of FIGS. 4A and 4B.
According to a second aspect of the invention, there is provided a method of producing a pulse width modulation (PWM) output signal train having fast transitions in a manner that less susceptible to PWM timing errors on account an interference signal imposed upon a control signal input thereto. As with the PWM circuit of the present invention briefly described above, this control signal has an analog value that is representative of the duty cycle of the desired PWM output. The method of the present invention comprises the steps of: (1) providing a first input signal connected to a triangular wave source; (2) providing a second input signal connected to the control signal representative of the duty cycle of the desired PWM output; (3) adding the first and second input signals in real time to produce a summed signal; (4) continuously providing the summed signal to an operational amplifier having a gain of at least five, in order to produce a substantially on-and-off output signal from the amplifier; (5) providing the substantially on-and-off output signal to a comparator; and (6) arranging the comparator to produce a PWM output signal that turns on and off in response to receiving the switching states of the substantially on-and-off signal.
As with the operation of the PWM circuit of the present invention, the method of the present invention further includes the steps of: providing a comparator that has first and second inputs, providing the substantially on-and-off output signal to the first input of the comparator, arranging the operational amplifier to produce a trapezoidal waveform as the form of the substantially on-and-off output signal, and providing the trapezoidal waveform to the first input of the comparator; and providing a reference signal to the second input of the comparator. Again, the operational amplifier is provided with a gain of at least five and preferably has a closed loop gain of at least ten, and more preferably has a closed loop gain of about 50 to about 100 or more.
The PWM circuit and PWM method of the present invention are each very well suited for driving a laser driver circuit. In such an arrangement, the input of the laser driver circuit is connected to the PWM output train of the comparator of the first or switching section of the PWM circuit of the present invention In that manner, the laser is turned on and off in step with and in response to the PWM output train, which in turn is accurately controlled by the level of the input control signal.